A fuel cell generally comprises two electrodes, namely an anode and a cathode, between which an electrolyte layer is arranged. Oxygen is passed over the cathode surface and a fuel is passed over the anode surface. Ion exchange between the fuel and the oxygen takes place via the electrolyte layer, so that a voltage is formed between anode and cathode.
One of the crucial factors with regard to the efficiency of a fuel cell is the electrolyte layer. On the one hand, it must have a good ion conductivity and on the other hand, it needs to be substantially gas-impermeable, in order to prevent gas exchange between fuel and oxygen. Therefore, high demands are imposed on the electrolyte layer.
In what are known as solid oxide fuel cells (SOFCs), the anode and the cathode are formed from a porous ceramic material, between which a solid electrolyte layer is arranged. Solid oxide fuel cells with a planar geometry, in which the anode and cathode run substantially plane-parallel to one another, are known. In addition, a cylindrical or tubular configuration is also known. A tubular fuel cell of this type comprises a porous ceramic inner cylinder as the cathode, to which the electrolyte layer is applied, followed by the anode as a casing. To produce electrical connection to the cathode, there is what is known as an interconnector, which is directly connected to the cathode, the electrolyte layer and the anode being interrupted in the region of the interconnector.
A large number of processes for applying the solid electrolyte layer are known. For example, DE 196 09 418 C2 discloses the application of a suspension which contains solid fractions of the solid electrolyte material to a planar electrode. Excess solvent is removed by reducing the pressure on the opposite side of the porous electrode from the suspension. The suspension has coarse and fine solid fractions, the coarse solid fractions initially blocking the pores in the electrode and ensuring good bonding between the electrolyte layer and the electrode. The fine fractions are then deposited on the coarse fractions. The solid layer is dried and then sintered in order to form the solid electrolyte layer. This coating process requires subsequent sintering of the electrolyte layer at a high temperature.
Moreover, DE 196 09 418 C2 discloses that it is known to produce the electrolyte layer by electrophoresis or by tape casting.
An EVD (EVD: electrochemical vapor deposition) process for applying the electrolyte layer is known from the article “Status of Solid Oxide Fuel Cell Technology” by S. C. Singhal, taken from: High Temperature Electrochemistry: Ceramics and Metals, 17th R isø International Symposium und Material Sience, Roskilde, Denmark, September 1996. This process is also suitable for complex surface geometries, in particular for the curved surfaces which are present in a cylindrical or tubular fuel cell. However, the EVD process is very expensive and complex.
It is known from the article that both the electrolyte layer and the interconnector and the anode are usually applied by the EVD process. The article deals with the problem of replacing the expensive EVD process with other coating systems. While new coating processes, for example plasma-coating processes, are proposed for the interconnector and the anode, the EVD process continues to be envisaged for the electrolyte layer in order to ensure that a sufficiently high quality is achieved.
One problem with the electrolyte layer is considered to reside in particular in the gastightness required. The solid electrolyte layer cannot generally be formed to be sufficiently gastight with a conventional plasma coating process, which is significantly less expensive than the EVD process. A plasma coating process usually applies a molten or pasty coating material, which then solidifies, to the substrate to be coated within a relatively small spray spot with a diameter of up to 4 cm. In the case of large-area components, for example in the case of the known tubular fuel cells, the small size of the spray spot means that the coating has to be applied in a plurality of tracks.
A plasma coating process of this type throws up in particular two serious problems for the formation of a solid electrolyte layer: firstly, the molten material shrinks as it solidifies, so that a porosity which corresponds to the degree of shrinkage is formed in the solid electrolyte layer. Therefore, a gastight layer cannot be achieved. Secondly, a process of this type is scarcely able to achieve homogeneous formation of the solid electrolyte layer with a uniform thickness, since the individual tracks overlap at their boundary regions. The fluctuations in thickness in the region of the solid electrolyte layer significantly reduce the efficiency of a fuel cell, since the ion conductivity across the solid electrolyte layer is adversely affected.
On account of the significant cost benefits of a plasma coating process, for example compared to the EVD process, more recent developments also aim to use plasma coating processes for the solid electrolyte layer. This is shown, for example, by the scientific article by Rudolf Henne “Potential of Vacuum Plasma Spraying for the Production of SOFC Components” and the article by Heiko R. Gruner et al. “SOFC Elements by Vacuum-Plasma-Spraying (VPS)”, both included in “Proceedings of first European Solid Oxide Fuel Cell Conference”, Lucerne, Switzerland, October 3 to 7, 1994, Volume 2, pages 617 to 627 and pp. 611 to 616, respectively. A common feature of both these articles is that they propose a vacuum plasma process, with a pressure of approximately 100 mbar being established in the coating chamber.
FR 2 729 400 A1 and the patent abstract to the Japanese patent application 63000450 respectively propose pressures in the range from 0.1 to 20 mbar and 10 to 100 torr (13 to 130 mbar).
However, even with these known plasma-coating processes, it is impossible to achieve a high-quality solid electrolyte layer which has the quality of a solid electrolyte layer produced using the EVD process.